In serial communication systems, a large percentage of the total power is consumed in the transmitter, which must provide for adequate signal swing on a low-impedance channel while maintaining an appropriate source termination. In addition, the transmitter often includes equalization to compensate for frequency-dependent loss in the channel. The driver circuit in the transmitter often consumes the majority of the power of the transmitter. Driver circuits can be implemented as current-mode drivers or voltage-mode drivers. Voltage-mode drivers are known to consume far less power in comparison to current-mode drivers. For example, a voltage-mode driver can consume four times less DC power than a current-mode driver to provide the same output swing.
A voltage-mode driver for a transmitter requires swing and impedance control such that the swing and common-mode/differential-mode return loss are within specifications. One technique for output signal swing control in a driver circuit is to use a single voltage regulator to generate a reference voltage that sets the voltage swing. However, with a single regulator, the common-mode will shift as the output swing of the driver circuit changes. Such a shift in the common-mode can cause the return loss to exceed specifications.